In recent years, the reflow method has been much used for soldering of electronic parts. The reflow method is a soldering method using solder paste. In this method, soldering is carried out by the steps of applying solder paste by printing and carrying out reflow heating. In the printing step of the reflow method, a mask having holes bored in locations corresponding to portions to be soldered is placed atop an object to be soldered, and solder paste is placed atop the mask and scraped with a squeegee to uniformly fill the holes in the mask with the solder paste. When the mask is then removed by pulling upwards, the solder paste which fills the holes in the mask is transferred to the portions to be soldered. This procedure is referred to as paste transfer. In the subsequent reflow heating step, after electronic parts are mounted on the portions of the object to which solder paste has been applied, the object is heated with a heating apparatus such as a reflow furnace to melt the applied solder paste for soldering. The reflow soldering method not only has excellent operability in that soldering of all the portions to be soldered can be carried out at one time but also has excellent reliability in that solder does not adhere to unnecessary locations.
Solder paste used in the reflow method is obtained by mixing solder particles and a pasty flux, and it has an appropriate consistency.
The state of recent electronic parts will be briefly described. With the trend towards multifunctionality and decreases in the size and weight of portable electronic equipment, chip-type parts are also becoming increasingly smaller, progressing from the 1005 size (10 mm×5 mm) to the 0603 size (6 mm×3 mm) and still further to the 0402 size (4 mm×2 mm). There is also a tendency for processing of semiconductors to move from wire bonding, which was formerly predominant, to flip chip mounting in order to attain decreases in size and high speed processing of signals. In advanced technological fields, there are increasingly severe demands concerning the quality, reliability, and price of parts. Cost is also becoming an important factor in flip chip mounting, and there has been a movement from gold bumps to solder bumps. Regarding solder bumps, a lot of research and development have been made on the stable bump formation by a method using solder paste, which is advantageous from a cost standpoint compared to solder plating or solder balls.
Normally, stable printing of solder paste and reliable paste transfer are desired when solder bumps are formed on a wafer by printing for minute surface mounting. In the formation of solder bumps on a wafer, the number of solder particles required to constitute one electrode is normally at least approximately 6 to 10 when they are arranged side by side. Assuming that an electrode formed on a 0402 size chip, has a diameter of 0.18 mm, it is necessary for solder particles to have a particle size which is obtained by a proper blend of #10 particles (5-15 micrometers in diameter) and #21 particles (15-25 micrometers in diameter). In the case of wafer bumps having a diameter of 100 micrometers, the upper limit on the diameter of solder particles is 10 micrometers, and in order to maintain stable paste transfer, a diameter of approximately 5-10 micrometers is desired.
Methods of manufacturing solder particles for use in solder paste include the atomizing method in which molten solder is dripped through a narrow nozzle into a drum and at the same time the resulting droplets are blown by high pressure gas to form fine particles (Patent Document 1), the rotating disk method in which molten solder is dripped onto a disk rotating at a high speed and is scattered by the centrifugal force of the rotating disk to form fine particles (Patent Document 2), and the agitation-dispersion method in which solder is charged into high temperature oil to melt, and the oil and the molten solder are agitated by an agitation-dispersion device to form fine particles (Patent Document 3). Solder particles obtained by these manufacturing methods for solder particles contain a wide range of large and small solder particles mixed together, with small ones having a size of several micrometers and large ones having a size of 100 micrometers or larger.    Patent Document 1: JP H 7-258707 A1    Patent Document 2: JP H 9-10990 A1    Patent Document 3: JP H 2-118003 A1